**Abstract**

This chapter shows how organic fertilizers can provide essential nutrients soluble to plants, so as to be used in hydroponic systems in its various forms. Such materials are an important source of macro- and micronutrients. This form of plant nutrition can contribute to the sustainable production of food, both in developed and developing countries. Nutrient solutions can be formulated when soluble nutrients are extracted from the solid phase of organic manure. In some vegetables, equal yields, or sometimes higher, have been obtained in nutritive solutions formulated with synthetic chemical fertilizers. It has also been documented that the resulting edible products can be of a better nutraceutical quality. Ions can be obtained by means of preparations based on teas, extracts, leachates, digestate, urine, aquaculture, etc. Subsequently they must be diluted in water until reaching a level of electrical conductivity according to the tolerance levels of the crop to be established. The heterogeneity of the chemical composition of the solutions obtained is the main point that must be attended with the greatest possible precision to formulate the nutritive solutions and obtain satisfactory results. Therefore, it is necessary to measure the concentration of macro- and micronutrients (NO3 <sup>−</sup>, NH4 + , SO4 = , H2PO4 <sup>−</sup>, K<sup>+</sup> , Ca++, Mg++, Fe+++, Cu++, Mn++, Zn++, Cl<sup>−</sup>) as well as the Na<sup>+</sup> ion (which is usually at high levels); it will also be necessary to adjust the pH. In addition, the chapter presents a broad overview and a series of research results in recent years: composition of solutions, nutrient supplements, substrates, and floating root trials in tomato, lettuce, cantaloupe melon, and green fodder. The environmental implications of inappropriate formulations are also analyzed. The nutritious solution, formulated from organic fertilizers, is not only an alternative for the nutrition of agricultural crops, but it also represents a more efficient way to use these resources.

**Keywords:** production systems, soilless, hydroponics, organic agriculture, plant nutrition

## **1. Introduction**

The cyclical dynamics of the elements allow their reuse in ecosystems but also in agroecosystems. Organic matter represents a phase where they are partially and momentarily retained to follow the flow to various destinations, such as soil. Possible sources of nutrients, derived from reused or recycled materials, include wastewater; sewage sludge; biosolids; animal manure; urban waste; compost; vermicompost; digestate; biocarbon; inorganic by-products such as struvite, ammonium sulfate, and food waste; agribusinesses; and other industries [1].

This chapter focuses on manure, which is often the most available in the world's producing areas and is an important source of macro- and microelements for plant. For example, global manure nitrogen (N) production increased from 21.4 Tg N yr<sup>−</sup><sup>1</sup> in 1860 to 131.0 Tg N yr<sup>−</sup><sup>1</sup> in 2014, with a significant annual upward trend (0.7 Tg N yr<sup>−</sup><sup>1</sup> , p < 0.01), according to estimates of Zhang et al. (2017). These authors mention that cattle dominated the nitrogen production of manure and contributed 44% of total manure nitrogen production in 2014, followed by goats, sheep, porks, and poultries. The application of nitrogen from manure to farmland accounts for less than one-fifth of the total nitrogen production of manure during the study period.

Manure nitrogen production is expected to increase in the coming decades due to the growing demand for livestock populations as a result of increased human populations and changes in the structure of the diet with higher meat consumption (Herrero and Thornton, 2013).

While, in each country, there are significant resources of organic materials as sources of plant nutrients, their commercial use in hydroponics may be feasible if there is high availability and affordable costs, and on the other hand, they must be accompanied by guarantee of safety and food safety. This production technique is very promising for food production and efficient use of water and nutrients.
